Create PULL_REQUEST_TEMPLATE.md

PULL_REQUEST_TEMPLATE.md

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+### Summary
+
+### Related Issues
+
+### PR Overview
+
+- [ ] This PR requires new unit tests [y/n] (make sure tests are included)
+- [ ] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
+- [ ] This PR is backwards compatible [y/n]
+- [ ] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)